The PS content in Hib conjugate vaccines in final containers is currently determined by measuring its ribose using colorimetric orcinol assay. However, the Merck vaccine contains a large quantity of lactose, a stabilizer, which makes the assay impossible; and an indirect rate nephelometry assay, which is based on the rate of antigen-antibody aggregation, is used to estimate the PS content of the vaccines. We have developed an HPLC method to quantitate the PS content in the vaccines. Our results show that the amounts of PS in the Merck vaccine as well as Connaught and praxis vaccines can be measured on a Dionex carbohydrate analyzer as follows. The PS in the conjugate vaccines was first depolymerized in 0.1 N NaOH at room temperature for six hours to its single repeating unit ( ribitol-ribose-phosphate ). The PS repeating unit in the alkali-treated sample was then separated from the huge lactose peak by an HPLC column and quantitated. This method can detect the PS as low as 0.1 mug or at a PS concentration of 1mug/ml.